Yes, it can be cut to a specific length. This is normal because not all 10ft ceilings are exactly 10ft tall. Aviation snips should be used to cut these - it will be very easy and a clean job. You will likely want straight snips for this, but if you have other applications, you might want left or right snips. This page has a good graphic on the uses of each. https://www.homedepot.com/p/Wiss-Straight-Cut-Aviation-Snip-3-Pack-M123R/100273810
Thanks for your inquiry. There are 50 pieces per box of the 10' galvanized corner bead.
Not recommend. Nails are best. Bugle head screw will not snug up tight without torquing the metal.
Yes, you want to use some sort of tape on inside corners.